Cannot install anything

Running Windows XP, been having multiple problems, resulting in a complete reboot a few months ago...
Now suddenly I cannot install anything. I didn't change anything, update anything... as a matter of fact, there is an update that for some reason WON'T install...
Cannot get WindowsXP CD to even run so that I can reinstall Outlook.
I updated windows installer, and it said that all was updated, but nothing.
Y'alls expertice would be greatly appreciated.

Just adding that I finally got an error that said that it couldn't find the MSIEXEC. How does that happen? I looked in my System32 folder in Windows, and it shows msi.old and msiexec.old... I am assuming that is not good. How do I fix this?

Grab your installation CD, go Start, run, type or paste in:
sfc /scannow -and press Enter. Follow the instructions, when finished the window will just close, no fanfare.
Actually, I don't know if that will copy in those files because they may not be listed as protected [but it won't hurt to try it]. If it does not you could then just copy those files off your CD [in i386 folder] into system32.
msi.dll has been updated recently. Windows update will pick that up though if you request it [the update website] to scan your system.

I have tried the sfc /scannow... nothing happened.
I put in the CD, I have to go into My Computer to get it to open, because i get an error message that says
Cannot find
c:/windows/system32/msiexec.exe
"the system cannot find the file specified"
when I try to run setup.

Can you not copy this and other missing files from the CD i386 folder?
c:/windows/system32/msiexec.exe
And you could always rename msiexec.old to msiexec.exe... mine is vsn 3.1.4000.1823.
mis.dll vsn is 3.1.4000.4039

It seems every time I download a program from the internet it causes a problem with my computer. I get a message that my computer is shutting down to avoid permanent damage. The message says to remove any new programs. When I do this my computer works fine again. Why does this happen?